1. Field of the Invention
This invention relates to sensing devices. It is particularly concerned with tactile sensors that can detect touch and detect and quantitate applied and reactive forces and applied force components normal to the sensor surface and the distribution of those forces on the sensor surface. Principal uses of such sensors are in robotics, teleoperation, industrial automation and prosthetics.
2. Prior Art
There has been a great deal of development effort in the past with regard to apparatus and devices, such as industrial robots and sub-systems for use with them, that will accomplish desired work objects. The field of industrial robots, for example, has expanded rapidly but further progress is presently limited by the absence of low cost adequate sensor perception and feedback control systems. Technology presently exists to permit design of robots able to respond to control signals and to move even as little as a few thousandths of an inch to accomplish their designed tasks. Such robots are usually engineered to accomplish specific jobs in well-defined work environments, and they are usually complex and costly.
A great deal of emphasis has previously been placed on artificial vision systems that will serve as "eyes" for a robot in the detection of objects or circumstances and that will provide signals for responsive operation of the robot. Consequently, the artificial vision systems have become quite sophisticated, while tactile sensors that have not received the same design emphasis have generally remained rather crude, often having coarse spatial resolution and slow response times.
Tactile sensors can synergistically complement visual systems by becoming the controlling system at the time contact is made between a gripper of the robot and an object or objects being gripped, this being a time when vision is often obscured. The potential importance of tactile information is evident when it is realized that the information can readily operate a robot hand to function in a manner comparable to the way a blindfolded person can sense objects and perform simple functions such as the threading of a nut onto a bolt.
It is believed that continuing advances in industrial robotics will require robots capable of performing tasks that will place a premium on the tactile sensory perception capability of the machines. It is also believed that relatively low cost touch responsive sensors are required for use with such robots. A number of motion control and force sensor devices have been heretofore disclosed. Representative of these devices are those shown in U.S. Pat. Nos. 3,307,393; 3,416,365; 4,098,001; 4,155,169; 4,156,835; and 4,094,192. These known devices may be useful for their specific design purposes and they are generally designed for use in complex mechanical systems, with the chief objective of the systems being to obtain high precision. However, because in most circumstances sensory feedback is lacking, the degree of precision obtained is not as high as desired.
Other force sensors have been developed, and when such individual force sensing elements are placed in an array, force distribution or tactile sensing can be achieved. One such design uses a sandwich structure of two linear arrays of electrodes separated by a thin material, the electrical conductivity of which changes with pressure. By constructing the array in such a manner that the two arrays of electrodes intersect diagonally, a force-sensitive cell is defined at the intersection. The dimensions of the cell are defined by the width and separation of the electrodes.
Another force sensor concept employs small "windows" that are arranged in a matrix. The "windows" are electrodes that are surrounded by a common grounding medium. When a conducting material covering the electrodes is deflected with sufficient force current will flow from the electrodes to the grounded material.
Still another variation of a force-sensor array consists of utilizing a sheet of silicone rubber and an etched circuit board. The rubber sheet is made of alternating conducting and nonconducting layers with the conducting layers being silver or graphite impregnated rubber. The conducting layers of the rubber sheet are placed in a perpendicular relationship to circuit board conductors. The contact points at each intersection then form pressure sensors. Complex data output and readout circuitry are required to obtain and use this force-sensor array.
Other devices utilize conducting rubber in association with a semiconductor material. The limitations of these devices center around the potential contamination of the semiconductor material as well as abrasion of the rubber surface. In addition, as with other devices utilizing conducting rubber, the response indicated between the deformation of the rubber and the pressure applied is often non-linear. In linearizing the response of the rubber, means are provided to digitize the pressure response. One method of linearizing employs a triangle-shaped device with a series of electrodes along two sides of the triangle. Progressively larger forces applied to a rubber surface resting on a corner of the triangle will cause the corner to penetrate into the rubber and to contact an increasing number of electrodes as the penetration continues and increases.
An optical device, made by the Lord Corporation of North Carolina, U.S.A. employs a small force-sensing device consisting of a small shutter that controls a light beam in conjunction with a light emitting diode and photodetector. Each element requires individual calibration.
While the prior art devices discussed above may each be suitable for a specific use for which it is designed, they are not widely adaptable for use because of their expense, complexity, nonlinear readouts and/or limited performance when used generally. It has been found, however, that an ultrasonic distance measuring device which measures the thickness of a compressible medium meets the general requirements necessary to provide a tactile sensor suitable for a wide range of uses. If a resilient medium is used, the device is adaptable for repeated uses.
The use of sound in the measuring of distance has long been known. In many applications ultrasonic pulse echoes or mechanical vibrations, are used for precision measurement of various properties of a given piece of material. Attention is directed to U.S. Pat. Nos. 4,033,244; 3,994,154; 3,688,565; 3,228,232; 3,108,469; 3,745,833; 4,044,606; 3,315,520; 3,540,265; 3,416,365; and 3,942, 381. As illustrated in these patents both thickness and acoustic properties of the material can be measured.